In my copending application Ser. No. 153,723, now U.S. Pat. No. 4,337,389 filed May 27, 1980, entitled "Glow Plug Control Device for Diesel Engines", there is described a thermally operated controller for a glow plug. The controller uses a thermally operated switch that is the thermal analog of the glow plugs to cycle the glow plugs on and off while maintaining the temperature of the glow plugs at a maximum safe operating level. A control circuit used in conjunction with the controller is described in U.S. Pat. No. 4,177,785. This arrangement requires a thermally operated circuit breaker that must be carefully designed to have a longer time delay than the controller in switching off power to the glow plugs and yet have a resistance that is less than the heater of the controller. In practice this imposes design limitations on the two switches that are very difficult to meet over a large ambient temperature operating range, so that at some ambient temperature, the circuit breaker may take too long to break the circuit to protect the plugs, while at some other ambient temperature, the circuit breaker may act before the controller to interrupt current to the glow plugs.